(1) Field of the Invention
This invention relates to a method and device for obtaining a determined flow resistance of a flow channel, in particular of an opening in a component.
This invention relates in general to the machining and sizing of flow channels, in particular of openings or mouths, and preferably of small openings, where it is necessary to achieve a critical flow resistance, as well as the correct balancing of flow resistances in a number of such flow channels.
(2) Description of the Related Art
The significance of the impedance of a flow channel is well known. Examples include, among other things, fuel injector nozzles, spray nozzles, the flow of cooling air through components of turbines, the dosing of lubricating oil for precision bearings etc. In many such applications an exact dosing of flows is of very great significance, but as a result of manufacturing limitations it likewise involves significant problems. Even very small differences in the manufacturing tolerances can cause major changes in the flow resistance and in the flow.
Moreover, parts are frequently cast or fabricated from a material that is selected for specific properties such as its conductivity or insulating action for heat or electricity, low weight, a coefficient of expansion during heating or cooling, cost etc., although there is a different set of requirements regarding the inside surface of the opening. These special requirements for the inside of the passage can be met by plating or coating with a metal that has the desired characteristics. Plating can be done by electroplating or electroless (autocatalytic) plating, while the coating can also be done by vacuum metallizing or the use of a carrier gas or another such technology. Electroless plating or vacuum metallizing is generally used to plate or coat the inner surface of castings, borings etc., where secondary cathodes for a uniform electroplating are difficult to place.
Parts with flow openings for a fluid are manufactured by a variety of casting and machining operations. For example, high-quality precision molding methods can be used for the fabrication of these parts. Nevertheless, there are certain differences in the dimensions of such parts, in particular with regard to wall thicknesses, which are due to minor alignment errors in the core or the result of a displacement of the core, as well as fluctuations in the surface characteristics, in particular the roughness of the surface, small pits, nicks, grooves, blisters or positive metal. In the extreme case, a very small crack in the core can result in a thin wall which projects into an inner passageway. All these factors can significantly alter the flow of the fluid.
Machining methods that are in current use, such as electrical erosion machining and laser drilling, or less common technologies such as drilling by means of an electron beam, electrical current and STEM drilling (an ECM technology that uses an acid fluid) are not sufficiently precise to prevent significant changes in the flow resistance. Even the most accurate of these methods, electroerosion machining, will not achieve a perfectly uniform flow resistance, because the length of an internal passageway can vary as a result of the machining method used, and can in turn cause fluctuations in the overall length of the hole and of the flow resistance, regardless of the uniformity of the diameter of the hole. Furthermore, non-uniform conditions are unavoidable in electroerosion machining and can result in variations in the size, shape, surface and condition at the edge of the hole.
Openings to be plated or coated must be sufficiently oversized to leave room for a corresponding thickness or the plating or of the coating, and the ultimate precision depends on the exact calculations for the plating or coating rates and on the accuracy of the drilling and plating operations. The product that can be achieved using current technology is not sufficiently uniform for most precision industrial applications. Thus there are restrictions on the choice of methods available to the manufacturer for the fabrication of the overall part from materials that have the properties desired for the opening or for the embedding of drilled parts with specified properties in castings that have been realized to receive them. These technologies have the precision problems related to drilling described above. The plating of openings that are bored into a material with metal that has different characteristics or even with the same metal, which results in a precision flow, opens up a whole new range of possibilities in the manufacture of many parts.
In many applications, the variances that are inherent to the drilling operations can be accepted within broad limits, and the related compromises regarding freedom of design, construction and performance are simply accepted as inevitable. For example, the delivery of measured quantities of fuel in internal combustion engines by pressure injection of the fuel requires the measured discharge of the flow through nozzles.
Greater accuracy in the regulation of the flow will make possible an improved utilization of the fuel, as well as increases in the economy and precision of the operation of the engine. The realization of fuel dosing systems of this type in current use is frequently based on the measurement of the actual flow resistance and a distribution of the inventories in ranges of flow parameters, to achieve an at least approximate matching of parts in an inventory within a range of deviation of specified tolerances. A method of this type is extremely complex on account of the significant inventory requirements. A significant number of parts also fall outside the range of the allowable deviations and must be reprocessed at great expense or rejected.
In the past, fuel injection nozzles were fabricated so that the critical dosing openings for the flow were formed by electroerosion machining, Because a number of components have flow channels that are becoming increasingly smaller and must be calibrated, i.e. set to a specified flow resistance, homogenization, essentially of the inlet edge of the flow channel, is becoming increasingly important, because the smaller the dimensions of the flow channel, the less appropriate mechanical machining methods become.
Another example in which the flow resistance of an opening is of critical importance is the creation of a cooling flow through gas turbine components such as turbine blades.
Turbine blades manufactured using precision casting techniques are typically cast or drilled (by means of laser drilling, STEM drilling or electroerosion machining) so that a number of holes are created that typically have a nominal diameter of approximately 0.3 mm to 0.8 mm and extend from the internal passageway to the vicinity of the leading edge of the profile, the trailing edge of the profile or any point along the blade profile.
To cool the blades, cooling air is displaced from the interior through the numerous holes into a current of high-temperature combustion gas. During this process, holes in the inner walls of the blades apportion the distribution of the cooling air. It is obvious that changes in the flow resistance can result in differences in the cooling action, which can lead to hot spots that alter the thermal equilibrium inside the components and the engine itself and can influence both the performance and the useful life of the components. The use of cooling air should be minimized, however, because the excessive use of cooling air reduces the efficiency of the engine by “stealing” energy from the compressor stage. When components of this type are used, a more precise control of the flow resistance of these passageways can result in a significant gain in efficiency of both the components and the units in which said components are integrated.
In addition to heads for fuel injector nozzles, spray nozzles, for the flow of cooling air through components of turbines and for the delivery of measured quantities of lubricating oil for bearings, there are numerous other uses of passageways or openings that are used to regulate or control a flow in which this invention can be used.
EP 0 441 887 B1 describes a method of the prior art for the treatment of openings to achieve a determined flow resistance in which a working fluid with which an opening is being machined flows through an opening and at a constant pressure the flow rate that varies during the machining is measured. In the alternative, the flow rate could be maintained constant while the varying pressure could be measured. As soon as the flow rate reaches a determined value or the pressure drops to a determined value, the machining process is interrupted. Of course, with this method flow resistances of the opening with regard to a fluid can be set accurately, although the specification of a constant pressure or a constant flow rate turns out to be complicated and time-consuming.
The object of this invention is therefore to create an improved method to achieve a specified flow resistance of a flow channel and a simplified device that is suitable in particular for the performance of the method, by means of which a flow channel of a component can be accurately calibrated with respect to its flow resistance and which specifically uses structurally less complex and expensive means than similar methods of the prior art.